1. Field of the Invention
This invention relates to an optical tap for coupling light into, or out of an optical fibre.
2. Related Art
In optical communications, the radiation used is not necessarily in the visible region of the electromagnetic spectrum, and so the words `optical` and `light` when used in this specification are not to be interpreted as implying any limitation to the visible spectrum. For example, the wavelengths preferred for transmission through silica optical fibres are in the infra red region, because the loss minima of silica fibres occur at 1.3 and 1.5 microns.
In many `in field` situations, it is necessary for an engineer to sample the signals that are being transmitted along an optical fibre, or to transmit signals to, for example, a telephone exchange. This may be for maintenance or testing purposes, or so that the engineer can receive and transmit communications.
The specification of our published patent application W088/07689 describes an optical tap that can be temporarily attached to an optical fibre, the fibre being bent, and the tap collecting light that couples out due to the local perturbation caused by the bend. With the arrangement described in that specification, the level of light that is coupled out exhibits a non-linear wavelength dependence. Moreover, a large proportion (98-99% of total core power), escapes from the core along optical paths nearly tangential to the fibre bend so this type of tap has a low coupling efficiency, and can only be used as an intrusive device. The reason for the high optical loss is that leakage occurs over the entire length of the bend, and continues for some distance afterwards, as fibre cladding modes formed by the presence of the bend eventually radiate out of the fibre.